The movements of the aircraft between a departure airport and a destination airport form the subject of a preparation leading to the formulation of a more or less intricate administrative document called the flight plan which brings together a set of details concerning the progress of the flight. This flight plan, drawn up, firstly, for the benefit of the air traffic flow control authorities (airports, air traffic controls, authorities, etc.), mentions, among other information, the identity and the type of the aircraft, as well as a brief definition of the scheduled route listing a chain of waypoints, linking the takeoff runway used at the departure airport to the landing runway scheduled at the destination airport, overflight constraints associated with the waypoints, the scheduled times at which the waypoints are to be overflown, as well as, possibly, the regulatory approach procedures followed on departure and on arrival and the air corridors followed.
The piloting of an aircraft is increasingly automated. It is performed by acting on the orientations of movable surfaces (aerofoils, flaps, etc.) and on the power of the engine or engines by way of actuators receiving position instructions formulated by equipment termed the “flight controls” so as to maintain the aircraft in a given attitude, prescribed by the pilot or an autofacility.
The flight controls constitute together with the actuators a first level of equipment which is distinguished from the other levels by the fact that it involves flight equipment indispensable to the pilot for acting on the aerofoils, flaps and engines. This first level of flight equipment is often supplemented with a second and third level of flight equipment which consist of an automatic pilot/flight director and of a flight management computer easing the task of the pilot and which are distinguished from the first level of flight equipment by the fact that the pilot could, in all strictness, do without it.
The automatic pilot/flight director eases the task of the pilot in following instructions relating to heading, altitude, speed, etc. It has two possible modes of operation: “flight director” operation where it indicates to the pilot, by way of viewing screens, the orders to be given to the flight controls so as to follow an instruction, and “automatic pilot” operation where it acts in addition on the flight controls for automatic following of the parameterized instruction.
The flight management computer acts on the flight controls by way of the automatic pilot/flight director. It ensures various functions described in the ARINC 702 standard (ARINC being the acronym of the company: “Aeronautical Radio Incorporation”) of December 1996 known as: “Advanced Flight Management Computer System”, including:                a function for inputting the brief definition appearing in the flight plan, for the scheduled route, that is to say of the chain of waypoints linking the takeoff runway used at the departure airport to the landing runway scheduled at the destination airport with the overflight constraints associated with the waypoints and the scheduled times at which they are to be overflown as well as the departure and arrival procedures and possibly the air corridors (or airways) followed,        a function for formulating a 3D trajectory taking the route to be followed defined briefly in the flight plan while complying with the performance of the aircraft and the flight constraints encountered along the 3D trajectory adopted, and        a guidance function generating, by way of the automatic pilot/flight director, piloting instructions and/or orders relating to the management of the thrust of the engines and of the aerodynamic configuration of the aircraft so as to follow the 3D trajectory formulated.        
During an approach phase preceding a landing, an aircraft descends in general from its cruising altitude to an intermediate altitude where it keeps a level deceleration, in the course of which it consumes its inertia until it reaches a speed compatible with a landing and aligns itself along the axis of the destination landing runway, on a descent plane allowing its wheels to touch down on entry to the runway. The trajectory of the approach phase as well as the speeds of traversal of the various portions of this trajectory often form the subject of a regulation termed the runway approach procedure defined by a series of waypoints which lead to the entrance of the chosen runway and which are associated with local flight constraints (altitudes, speed, etc.).
The flight management computer, when it has been parameterized at the start of a mission with a flight plan comprising an approach procedure for the destination landing runway, can, once its guidance function has been activated, ensure the guidance of the aircraft in the course of this approach phase, by providing the automatic pilot/flight director with the orders necessary for, on the one hand, reducing the speed of the aircraft while progressively altering its aerodynamic configuration (extension of the flaps, lift-enhancing slats, etc.) so as to retain its lift and maintain its stability at low speed and, on the other hand, passing the waypoints imposed by the regulatory approach procedure while complying with the local flight constraints associated with them.
However, it may happen that in the course of a landing field approach procedure, an aircraft receives, from the authority in charge of regulating the traffic in the airspace of the destination airport, a speed instruction not scheduled in the flight plan, justified by the necessity to insert it into the local traffic. The pilot of the aircraft must then resume control of the piloting of the aircraft if he had entrusted it to the flight management computer, and apply the speed instruction of the air traffic controller in addition to the instructions passed to him by the flight management computer through the automatic pilot/flight director, which are displayed on the onboard instruments and which are not necessarily compatible with the speed instruction of the regulating authority. This results in extra work for the pilot in a phase of the flight where the workload of the crew is already considerable. There therefore exists a requirement to ease the handling by the crew of an aircraft, of the speed instructions that they receive while they are getting ready for or are currently executing a landing runway approach procedure.